The grip that tires have on the ground on which they are rolling is one of the most important features from the point of view of the safety of the driver of a vehicle fitted with tires. It is also of key importance in determining the performance of the vehicle in sporty road driving. If its tires lose their ability to steer as a result of a lack of grip, the vehicle can no longer be steered.
Of course, a vehicle, even if designed for sporty use, has to be driven in variable weather conditions. It is, therefore, a known practice for the tire to be provided with means that provide good grip on dry ground and on wet ground. In particular, it is possible to adapt at least part of the tread pattern to use on wet ground, for example by providing recesses able to drain away and/or to store water, or by increasing the number of tread pattern edges able to cut through the film of water formed between the tread and the ground. It is also possible to vary the materials of which the tread is made, using rubber compounds more particularly suited to use on wet ground and/or on dry ground. A tread comprising the two types of rubber compound is able to achieve good grip under all circumstances. An example of such a tire is given in document EP 1 308 319.
Under sporty road driving conditions, the tires of a vehicle experience substantial transverse stresses when the vehicle fitted with the tires is cornering. During the corner, the transverse stresses cause, on the contact area where each tire makes contact with the ground on which it is rolling, deformation resulting in a substantially trapezoidal shape. The side of the contact area which is furthest away from the center of the bend lengthens, while the side of the contact area closest to the center of the bend shortens.
The “side of the contact area furthest from the center of the bend” is the side via which the elements of the tread come into contact with the ground in the direction of the rate of drift of the center of the wheel on which the tire is mounted. For this reason, it is sometimes referred to as “(transverse) leading edge”. The opposite side, that is to say the “side of the contact area closest to the center of the bend” is sometimes referred to as the “(transverse) trailing edge”.
This “trapezoidal” deformation alters both the load borne by the various ribs of the tread and the contribution that each makes to the transverse force developed by the tire. For a given load that the tires of the vehicle have to support in a given cornering situation, the ribs that have become lengthened bear a greater share of the total load borne by the tire. The ribs which have shortened bear a correspondingly lower proportion of the total load borne by the tire. For a given transverse force, delivered by one of the tires in a given cornering situation, it follows that the most heavily loaded ribs (in general, those on the side furthest from the center of the bend) are those which make the greatest contribution to the total transverse force.
Rubber compounds suited for use on wet ground are generally more fragile with respect to the very high thermal and mechanical stresses generated in the contact area of a tire under severe cornering conditions on a dry road surface. If the tread of the tire is provided with portions made of a rubber compound with better grip on dry ground and with portions made of a rubber compound with better grip on wet ground, then it is preferable to ensure that the rubber compound that has better grip on dry ground is placed on the side of the contact area that is furthest from the center of the bend. Thus, even if the contact area becomes trapezoidal, the tire will maintain good grip on dry ground, that is to say a good ability to develop a high transverse force. Further, because the ground contact pressures are higher on this same side of the contact area (which is the furthest from the center of the bend), the drainage of the water with which the road surface is wetted is generally rather satisfactory in this part of the contact area. As a consequence, conditions that establish good grip contact and that allow use of a rubber compound with better grip on dry ground are created in this region of the tread. In other words, the tire, in this region, behaves as if it were rolling on dry ground. There is therefore no need to provide in this part of the tread a rubber compound that has better grip on wet ground and of which the performance on dry ground is inferior than that of a rubber compound that has better grip on dry ground. The “Pilot Sport 2” tire commercialized by Michelin is an example of a tire that has such a distribution of rubber compounds within its tread.
In spite of the good performance offered by this tire in terms of grip, there is still an increasing need to improve the compromise between grip on dry ground and grip on wet ground of tires, and more particularly of tires designed for sporty road driving.